How Can I Check If the User Authenticated To Access Web Pages From Another Source

I am going to make web application using PHP which contains many pages, but I should check if the visitor is authenticated to view content or not by X domain.

The X domain can read the header MSISDN from the visitor (if exist) and check if he is authenticate or not.

So I am going to redirect user to X domain to allow him to read the header, and he will redirect user back to me if it can access content, what is the best way for X domain to inform me if the user is authenticated or not!

As I see:

1. I will not use a GET parameter that its not secured anymore.

2. I tried to check referring URL but I could not always read it!

is it good way to X site send me a POST parameter, or their are another method?

Noting that its not too important to be very secured.